Breakdown voltage provides an indication of the ability of a semiconductor device (e.g., a metal oxide semiconductor field effect transistor (MOSFET) device) to withstand breakdown under reverse voltage conditions. To realize an energy efficient power conversion system, power MOSFETs (e.g., MOSFETs designed to handle medium to high voltage levels) should have low conduction losses. Conduction losses can be lowered by reducing RDS(on), the on-state resistance between the drain and the source. However, reducing RDS(on) adversely affects breakdown voltage.
The drift region in a MOSFET is a relatively high resistivity layer grown by epitaxial (epi) technology, and is designed to achieve particular values for electrical characteristics such as breakdown voltage and on-state resistance. For medium voltage (e.g., 100 V) to high voltage (e.g., 600 V) devices, the major portion of the on-state resistance comes from drift region resistance. For example, for a 200 V device, an analysis shows that 88 percent of the total on-state resistance is due to drift region resistance, while only six percent is due to channel resistance, five percent is due to package resistance, and one percent is due to substrate resistance. Consequently, reducing drift region resistance can make a significant contribution to reducing the total on-state resistance.
However, while a reduction in resistivity in the drift region of the epitaxial layer can positively affect RDS(on), conventionally such a reduction means that breakdown voltage would be expected to be negatively affected as noted above.
Accordingly, a semiconductor device (e.g., MOSFET) that provides reduced resistivity in the drift region and hence lower on-state resistance, but does not negatively impact breakdown voltage, would be valuable.